The present invention relates to a miniature switch having a self-aligning displaceable elongated contact arm. Preferably, the switch is a thermostat.
A DIP (dual in-line package) is an electronic component available in several standard sizes and includes a housing having downward extending terminal pins which are received in plated through holes of a PCB. The chip or the like in the housing is thus connected to circuitry and other components.
U.S. Pat. No. 4,620,175, which is incorporated herein by reference, discloses a simple thermostat configured as a standard DIP (dual inline pin, 0.4xe2x80x3 wide, with pins on 0.1xe2x80x3 centers) having four terminal pins. Since the package is virtually identical to a standard DIP, it does not require any special hole spacing by the PCB manufacturers or any special assembly equipment. The terminal pins come in two pairs, the pins in each pair being connected through a single inward end inside the housing so as to be electrically redundant.
The housing for the thermostat of U.S. Pat. No. 4,620,175 includes a floor, opposed parallel sidewalls, and opposed parallel endwalls, that define a cavity therebetween. The housing is molded onto first and second terminals, discussed above, so that the inward ends are exposed at opposite ends of the floor and the pins extend downward therefrom through the housing. A contact arm is fixed to the inward end of the first terminal and has a cantilever arm whose free end has a contact fixed thereto and is biased away from but movable toward a fixed contact on the inward end of the second terminal. A fulcrum plate having a central dimple is fixed between the sidewalls and a bimetal strip is located between the fulcrum plate and the contact arm. The bimetal strip has two stable positions, and is thus considered to be bistable; in a first stable position the strip is bowed convexly against the dimple and biases the contact on the contact arm against the contact on the second terminal, to complete the circuit between the first and second terminals. In the second stable position, which occurs when the bimetal strip rises above a predetermined temperature, the bimetal strip is bowed concavely toward the dimple so that the contact arm springs away from the second terminal to open the circuit. The foregoing describes an xe2x80x9copen on risexe2x80x9d thermostat; a xe2x80x9cclose on risexe2x80x9d device can be provided simply by inverting the bimetal strip.
The DIP thermostat of U.S. Pat. No. 4,620,175 is intended for mounting in a region having an airflow sufficient to activate the switch through conduction of heat to and from the ambient air.
A board mounted thermostat is disclosed in U.S. Pat. No. 4,795,997, expressly incorporated herein by reference, is virtually identical to a TO-220 package as depicted in FIG. 1. This package has but two pin terminals and further has a mounting bracket fixed thereto. The terminals are flat stamped pieces to which the housing is molded. The thermostat described in U.S. Pat. No. 4,795,997, includes a cover plate incorporating the mounting bracket fixed over the cavity, which provides means for efficient heat transfer to the thermostat, to assure timely operation and thus protect the system. Thus, while a standard TO-220 electronic package uses the mounting bracket to dissipate heat from a semiconductor chip or resistor, the thermostat design uses the bracket to conduct heat between a nearby surface to the thermostat, so that the thermostat senses the temperature of the surface.
One problem with the design according to U.S. Pat. No. 4,795,997 is that, during volume production, there are a relatively high number of reject thermostat assemblies, resulting from contact spring misalignment.
It is an object of the present invention to improve miniature thermostat designs, by providing a switch contact arm having a self-aligning feature with respect to a switch housing. This feature preferably results in a good production yield.
It is also an object according to the present invention to provide a contact arm having a base and an extension, wherein the base comprises a form fit or force fit configuration for aligning the extension with respect to a base mounting space.
The present invention preferably provides improved quality, such as by limiting rejected parts due to a spring misalignment failure mode.
The present invention provides a thermostatic switch for mounting to a circuit board, having a housing configured with a cavity, with first and second terminals formed within the cavity, one of the terminals being displaceable with respect to the other, selectively forming a conducting path between the two terminals. The displaceable terminal is configured as an elongated cantilever arm, extending from a base. The base is form or force fit into the cavity and aligned by boundary walls thereof, causing the displaceable elongated cantilever arm to be aligned with the other terminal. The base preferably comprises a pair of bent tabs which contact opposite lateral sidewalls of the cavity.
These and other objects will become apparent from a review of the detailed description of the preferred embodiments of the invention.